EVD Scale in mmHg and cmH₂O: How to Convert and Read Both Units

An EVD measuring scale shows both mmHg and cmH₂O because a drainage level is a height of fluid, naturally read in cmH₂O, while intracranial pressure is usually charted in mmHg. To convert, 1 mmHg ≈ 1.36 cmH₂O and 1 cmH₂O ≈ 0.74 mmHg. The 30 cm scale on DESU’s EVD-010 and EVD-020 carries both units; the level itself is set by the clinician’s order and the IFU.

ICU nurses, theatre staff and educators work with both units every day, so the sections below cover where the two units come from, a quick conversion table and the places where unit mix-ups tend to happen. Both DESU systems are described on our EVD sets page.

Why does an EVD scale show two units?

A gravity-based external drain works by height. CSF drains when the pressure in the ventricles exceeds the height of the drip chamber above the reference point. That height is measured in centimetres, and because CSF is very close to water in density, a height in centimetres reads directly as cmH₂O.

Intracranial pressure monitors, on the other hand, report in mmHg, the same unit used for arterial blood pressure. That lets teams calculate cerebral perfusion pressure by subtracting one mmHg value from another. So the same patient may have an ICP in mmHg on the monitor and a drain level in cmH₂O on the pole.

Hospitals and countries also differ in habit. Some write drain orders in cmH₂O, others in mmHg. A scale that carries both lets the nurse read the value in whichever unit the order uses, instead of converting at the bedside.

Where does the 1.36 factor come from?

Both units describe the height of a liquid column. Mercury is about 13.6 times denser than water, so a column of water must be 13.6 times taller than a column of mercury to exert the same pressure. One millimetre of mercury therefore equals 13.6 millimetres of water, or 1.36 cmH₂O. Going the other way, 1 cmH₂O equals about 0.74 mmHg.

• mmHg to cmH₂O: multiply by 1.36.
• cmH₂O to mmHg: multiply by 0.74, or divide by 1.36.

What does a quick conversion table look like?

The table below converts the same round numbers in both directions, rounded to one decimal place. The values are arithmetic, not clinical targets.

Notice how far apart the two columns drift. A bare “15” means 20.4 cmH₂O if it was meant in mmHg, or 11.0 mmHg if it was meant in cmH₂O. That gap is why the unit matters more than the number.

Does rounding the factor matter?

Rounded factors are tempting at the bedside. Using 1.3 or 1.4 instead of 1.36 shifts the result by more as the value grows: 20 mmHg becomes 26 cmH₂O with 1.3, 27.2 with 1.36 and 28 with 1.4. Whether a difference of that size matters is a clinical question, but there is no reason to introduce it. A unit reference card using one agreed factor, or a scale that already shows both units, removes the temptation.

Where do unit mix-ups happen?

In practice, the arithmetic is rarely the problem. The errors come from how numbers travel between people and documents.

• An order written as a number without a unit.
• An ICP value read from the monitor in mmHg and compared directly with a drain level in cmH₂O.
• Reading the wrong side of a dual-unit scale, especially in low light.
• Mental conversion under time pressure, or with a different factor than colleagues use.
• Handover between staff, units or hospitals that use different conventions.
• Re-levelling after the patient has been moved or transported, when the reference point must be found again.

Units and levelling are closely linked. A correct unit read against the wrong reference point is still wrong. Our article on EVD zeroing and levelling explains the reference point and how it moves with the patient.

Who decides the drainage level?

The measuring scale is a reference, not a recommendation. The drainage level, the unit it is expressed in, the reference point it is measured from and how often it is checked are set by the clinician’s order, the instructions for use and the unit’s protocol. Nothing on the scale itself tells staff what the level should be.

Why the level matters, and what happens when drainage is too fast or too slow, is covered in our article on CSF overdrainage and underdrainage.

How should units be documented?

• Write the unit next to every number, in orders, charts and handover notes.
• Agree one unit for drain orders across the unit, and say which in the protocol.
• If a conversion is unavoidable, record the original value and unit next to the converted one.
• Use one conversion factor, printed on a reference card at the bedside.
• When in doubt about an order, ask the prescriber rather than convert and guess.

How do DESU EVD-010 and EVD-020 present the units?

Both systems share the same drainage components, including a 30 cm measuring scale that shows pressure in mmHg and cmH₂O, a 150 ml drip chamber and an 800 ml air-filtered drainage bag. The EVD-010 Series is supplied with a 31 cm ventricular catheter; the EVD-020 Series is used with a ventricular or a lumbar catheter. Because the scale is identical across the two systems, staff read units the same way whichever system is in use.

Talk to a DESU product specialist

If you are preparing a nursing in-service, a bedside reference card or an evaluation of EVD-010 or EVD-020, our product specialists can provide documentation and samples. Write to sales@desu.tr, use the contact page, or request a quote.